Radioreceiver



July 249 1934., P. o. FARNHAM RADIORECEIVER Filed May l8. 1955 www,

ma.. Q. NIPN Patented July 24, 1934 UNITED STATES PATENT OFFICE RADIORECEIVER Application May 18, 1933, Serial No. 671,771

13 Claims. (Cl. Z50-20) This invention relates to radio receivers and is directed particularly to the problem of manually controlling the output level of receivers of the automatic gain cont'rol type. The invention is especially useful in the further event that such manual level control is placed at a considerable distance from the audio detector, audio amplifier, and electro-acoustic transducer, as in the case of a remote-control receiver.

Prior systems for the remote control of the room level of reproduction have been comparatively expensive and complicated or have lowered the tone quality. In modern radio receivers the sensitivity is made to vary automatically in opposite sense and approximate inverse proportion to the strength of the incoming carrier wave (automatic gain control). Regardless of the completeness or eiciency of such automatic gain control operation, it is obviously desirable that the magnitude of the audio-frequency currents delivered to the electro-acoustic transducer be independently under the operators control.

This manual control has hitherto usually been provided by an adjustable coupling or transferlimiting element such as a voltage divider in one of the audio frequency circuits and less commonly by a manual control of magnetic coupling in a radio stage feeding the audio detector, since with existing tubes and the high audio voltage levels commonly used in modern receivers it is inadvisable to control the conductance of a tube used to amplify audio voltages in view of the distortion thereby introduced.

Such adjustable controls of audio level as were employed have certain disadvantages especially in remote controlled receivers. Since the usual voltage divider type should be placed at a point in the audio amplifier at which the voltage level is as low as possible, the pickup of noise or hum 40 voltages occasioned by the exposures created in running lines from the audio amplifier to a remote A control point is undesirably high. The characteristics of such lines (as in a cable for example) would also require the use of low im- 45 pedance terminations for the audio voltage divider or attenuator to preserve the transmission of high audio frequencies and this procedure is expensive. In some instances, a motor at the receiver is used to actuate the output level control 50 and is operated from the remote point, but this method is evidently a costly one. Other problems have been encountered with the types of audio level control previously used due to undesirable changes in iidelity with level settings and to the necessity for eliminating the coupling to circuits at the input of the radio amplier of radio voltages inadvertently appearing on the control leads.

An object of the invention is to provide a radio receiver including a simple and effective manual control of the receiver sensitivity and output. An object is to provide a manual control system of a form which is equally well adapted for incorporation in a receiver of the usual single unit design or in the control unit of a receiver of the remote 55 control type.

A further object of the invention is to provide a simple and effective manual control for adjusting the output level in radio receivers of the type in which the amplifier gain is automatically controlled before demodulation of the received signal. More particularly, anobject is to provide a radio receiver of the automatic gain control type and including a radio frequency amplier stage following that point in the transmission line at which the radio voltage controls the operation of the gain control system, the amplifier stage including adjustable circuit elements, which may be located at a point remote from the tube of the ampliiier stage, for adjusting a direct current bias on the tube, thereby to control the sensitivity and output of the receiver.

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompanying drawing in which:

Fig. 1 is a functional diagram illustrating graphically the method of operation contemplated by the invention, as applied to a superheterodyne receiver; and

Fig. 2 is a fragmentary circuit diagram of a receiver embodying the invention.

As indicated graphically in Fig. 1, modulated carrier waves picked up by the antenna are selected and ampliiied in the carrier frequency am- 95 pliler 1 and are mixed in the usual way with the local oscillations in an oscillator-first detector system 2 to produce intermediate frequency energy, the carrier frequency coupling between these sections of the receiver being indicated by 10o the dotted line arrow 3. An intermediate frequency coupling 4 joins the network 2 to a selective network 5 which may include an intermediate frequency amplier, and two intermediate frequency paths 6, 7 couple the selective network 105 5 to a control stage 8 and an automatic gain control system 9, respectively. The gain control system 9 includes an intermediate frequency amplier and rectifier, the direct current output of the rectifier being returned, by path 10 to 110 the carrier frequency amplifier 1 and oscillatorfirst detector system 2 to control the receiver gain automatically in advance of' the intermediate frequency path 7. A portion of the direct current output may be fed to the control stage 8, by path 1l, to control the gain of that stage automatically, in accordance with the invention described and claimed in the copending application of Christopher J. Franks, Ser. No. 600,750, filed March 23, 1932. This latter connection provides a means for maintaining a substantially constant radio output level at the intermediate frcquency coupling 12 to the detector-audio amplifier system 13. The radio voltage level at paths 6, 7 must rise somewhat with increasing signal strength to provide the necessary activity for developing a gain control voltage, at paths 10, 11, which varies with signal strength, but the automatic control of the gain in stage 3 compensates for the voltage rise at the input to stage 8.

Under the conditions thus far specified it will be seen that the transmission of the control stage may also be affected by a manual adjustment of bias on the tube included in stage 8 such as, for instance, an adjustment of a self-biasing resistor in the cathode circuit or an adjustment of screen grid or other polarizing potentials, as is indicated by control unit 14 and the direct current path 15.

With a suitable tube of the variable-mu type in the control stage the transmission may thus be varied over wide limits even though the radio voltage level at coupling 6 be fairly high, say Aof the order of one volt, without introducing appreciable distortion in the system. In this way, there is provided a method for distortionless control of audio output level by means of a simple direct current biasing circuit. The advantages for remote control operation are at once obvious. The direct current control line 15 may be of relatively high resistance and may be by-passed as heavily as desired Without effect on fidelity to prevent undesirable voltages being picked up thereon. It is apparent that the range of control required to obtain extinction of output is not excessive since the automatic gain control unit 9 holds the radio voltage level at coupling 6 fairly constant. The control unit 14 thus provides a means for manually adjusting the audio amplifier output which is transferred to the reproducer 16 by the audio frequency coupling 17.

The particular receiver circuit illustrated in Fig. 2 is divided physically into a remote control unit A and a power supply and reproducer unit B which are connected by a cable C in which no frequency higher than an intermediate frequency is transmitted.

'I'he remote control unit A contains the radio amplifier tube 20 and an oscillator-detector tube 21, which may be of the type known commercially as the 6A7. Intermediate frequency voltage from the anode circuit of tube 21 appears across the tuned circuit 22 which is coupled with approximately critical coupling by means of lead ain the cable C and coupling condenser 23 to the two coupled circuits 24 at the other end of the cable, in accordance with the invention described and claimed in my copending application.

So far as the present invention is concerned, the particular construction of the carrier frequency circuits of tubes 20, 21 and the oscillator circuit of tube 21 may be of any suitable design, the only requirement being that the low potential terminals of the carrier frequency circuits be isolated from the direct current potential of the cathodes to permit the application of an automatic gain control voltage to tubes 20, 2l through the lead b of cable C. The antenna or collector structure 25 is preferably associated directly with unit B and connected to the coupling coil 26 through the lead c of the cable.

The tuned circuit 22 and coupled circuits 24 of Fig. 2 correspond to the selective network 5 and coupling 6 of Fig. l, and serve to impress an approximately constant radio voltage upon the amplifiers tube 27 which constitutes the tube of the control stage 8 of Fig. l. Tube 27 is preferably of the pentode type and the cathode circuit of the tube in'cludes a fixed resistor 28 which determines the minimum negative bias on the control grid of the tube 27 when the manual control resistance 14 is adjusted to zero value.

The resistor 28 is not connected directly between the tube cathode and ground but, through lead d of cable C, is connected to ground through the manually adjustable resistance 14' in the control unit A. The junction of resistances28 and 14 is connected through a resistance 29 to a point of high direct current potential which, as indicated by the legend, may be 250 volts positive with respect to ground. Resistance 29 is of such magnitude that, when resistance 14' is adjusted to its maximum value, the combined voltage drop across resistance 28 and the voltage established through resistance 29 places a high negative bias, say of the order of 50 volts, on the control grid of tube 27. When resistance 14 is adjusted to zero value, the bias on tube 27 is determined solely by the resistor 28 and may be about 3 volts negative with respect to the cathode.

The heaters of tubes 20, 21 are fed in series from an appropriate source through the leads e, f of the cable, the source being indicated as a secondary winding 30 of a power supply system housed in the unit B of the receiver. The particular tuned network 'associated with tubes 20, 21 will not be described in detail as it is believed that the illustrated circuits will be understood by any person familiar with the art.

The gain control unit of Fig. l is constituted by the combined amplifier-rectifier tube 31 and its associated network, the coupling 7 to the intermediate frequency selective network including a small condenser 32. Tube 31 preferably takes the form of a pentode amplifier having an anode or anodes A which cooperate with the tube cathode to constitute a diode rectifier.

As illustrated, the radio voltage impressed on grid G1 of tube 3l is amplified by tube 31 and the resulting voltage is transferred to the diode rectifier AK by the transformer 32. The direct current voltage developed across the output resistance 33 is impressed upon lead b of cable C through the filter formed by resistance 34 and condenser 35. A desired fraction of the direct current voltage across resistance 33 is impressed upon the grid circuit of control tube 27 through the filter formed by resistance 34 and condenser 35.

The coupling between the control stage and the demodulator-audio amplifier tube 36 is provided by the coupling transformer 12 which has a secondary connected, in series with an output resistance 37, between the joined anodes A and the cathode K. The audio voltage developed across resistance 37 is impressed onV the control grid G of tube 36 through the filter formed by resistance 37 and intermediate frequency bypass condenser 39. The plate P of tube 37 is connected directly to a source of positive potential, indicated by +B, and the amplified audio voltage is deage on the diode rectifier exceeds this negative bias. Appropriate bias on'the control grid G of tube 31 is provided by connecting the grid through a high resistance 44 to a point on the cathode resistor 43 which has a direct current potential which is' about 3 volts negative with respect to the tube cathode.

The amplifier elements of tube 31 therefore provide a high amplification of the radio voltage prior to rectification. If slightly less selectivity is desired ahead of the automatic control system, the coupling condenser 32 may be connected to the high potential terminal of the first of the coupled intermediate frequency circuits 24.

It will be noted that the detector, i. e., the diode elements A, K of tube 36, is of the type which has a substantially linear output-input relation even at low input levels, thus avoiding distortion when the manual control resistance 14' is adjusted to give a low audio output. Although it is not possible to avoid distortion when the output level of a radio receiver is controlled solely by varying the bias voltage on a single radio amplifier tube, the described circuit does provide a high quality of reproduction since only acomparatively small range of control is imposed upon the manual control stage. This condition arises from the fact that the automatic gain control holds the radio voltage input to the control tube 27 approximately constant and thus, for any adjustment of the control resistance 14', the radio voltage delivered to the detector is substantially constant for a wide range of antenna input level. With approximately constant radio output at the manual control tube 27, the required range of variation of output level at'the speaker falls with the range of amplification variation which may be obtained, and without introducing distortion, by adjustment of the manual control resistance 14'. When the control resistance 14 is adjusted to its maximum value, the direct current potential impressed on the cathode circuit through resistance 29 places a high negative bias on the control grid and blocks transmission through the tube.

When resistance 14' is adjusted to zero value, the bias on tube 27 is determined by resistance 28 and by the automatic bias suppliedv through connection 11. Whether orl not any control voltage is supplied by the automatic gain control, the bias on tube 27 is reduced to that value which will provide the predetermined maximum output voltage at the reproducer when the effective resistance of the control unit 14 is reduced to zero.

The-advantage of the described manual control is that the adjustable unit 14 may be spaced from the receiver by a simple and inexpensive cable connection. The cable leads carry only direct current and therefore no distortion is introduced by the remote control as the leads may be heavily bypassed for radio and audio frequencies.

I claim:

1. In a radio receiver, the combination with a first amplifier for amplifying a modulated radio voltage substantially without demodulation, and automatic means for varying the gain of said amplifier substantially in inverse proportion to the strength of the carrier component of the modulated radio voltage, said means including a rectifying system having input terminals upon which the amplified output of said first amplifier is impressed and output circuit connections for impressing the direct current output voltage'of said rectifying system upon said first amplifier as a gain control bias, of a second amplifier for amplifying substantially without ldemodulation the modulated radio' voltage output of said first amplifier, means adjustable manually to control the gain of said second amplifier, and a demoduiator succeeding said second amplifier.

2. A radio receiver as claimed in claim 1, wherein said rectifying system includes a rectifier for producing a direct current bias voltage by rectification of an amplified radio voltage proportional to that developed by said first amplifier, and means for rendering said rectifier inoperative until the radio voltage input to said rectifier exceeds a predetermined level.

3. A radio receiver as claimed in claim A1, in combination with means for impressing upon said second amplifier a fraction of the said direct current potential.

4. In a radio receiver, a plurality of cascaded radio frequency amplifier tubes, a detector and audio amplifier, automatic gain control means having an input circuit connected to the output circuit of a tube in advance of at least one of said cascaded tubes, circuit connectionsfor returning a gain control voltage from said gain' control means to an amplifier tube preceding said output circuit, and manually adjustable means for controllingthe voltage level at the output of said audio amplifier, said adjustable means comprising means for adjusting the direct current bias on a. cascaded radio amplifier tube subsequent to said output circuit.,

5. A radio receiver comprising a plurality of cascaded radio'frequency amplifier tubes, a detector and an audio frequency amplifier, automatic gain control means and a last radio frequency amplifier connected in parallel across the last of said cascaded tubes, circuit connections for returning the -rectified output of said gain control means to said cascaded tubes to control the gain thereof, said last radio frequency amplifier having an output circuit coupled to said detector, and means for applying an adjustable direct current bias voltage to said last radio frequency amplifier to control the output level of said receiver.

6. A radio receiver as claimed in claim 5, in combination with circuit elements for impressing uponl the said last radio frequency amplifier a fraction of the rectified output of said gain control means, thereby to reduce the variation of detector input with variation in the strength of received signals.

7. A radio receiver as claimed in claim 5, Wherein said bias applying means includes an adjustable resistance in the cathode circuit of said last radio frequency amplifier.

8. A radio receiver as claimed in claim 5, wherein said bias applying means includes `a fixed bias resistor connected to the cathode of said last radio frequency amplifier, a manually adjustable Aresistance connected between said resistor and ground, and means connected to the junction of said resistance and resistor to esl tablish thereon a positive direct current voltage.

9. In a radio receiver, the combination with a carrier wave amplier, a local oscillator, a first detector, an intermediate frequency amplifier, and means automatically controlling the gain of one of said ampliers in accordance with and in a sense opposite to changes in the radio voltage level at the output of one of said amplifiers, of a demodulator, and a control stage having intermediate frequency input and output circuits coupling said control stage between said intermediate frequency amplifier and said demodulator, said control stage including an amplier tube and means for manually controlling the gain of said tube.

l0. A radio receiver as claimed in claim 9, wherein said manual control means comprises an adjustable resistance in the cathode circuit of said tube.

11. A radio receiver as claimed in claim 9, wherein said manual control means comprises an adjustable resistance at a point remote from said tube, and cable means connecting said resistance between the cathode of said tube and ground.

12. A radio receiver as claimed in claim 9, wherein said manual control means comprises a xed resistor and an adjustable resistance serlally connected between the tube cathode and ground, the xed resistor having one terminal connected to the cathode, and a resistance connected between the opposite terminal of said last resistance and a source ,of direct current potential that is positive with respect to ground.

13. A radio receiver comprising a plurality of cascaded radio amplifier stages working into a detector, manually adjustable means for controlling the voltage level at the output of said detector, and means operative at all adjustments of said manually adjustable means to maintain the output radio voltage level of an intermediate one of said cascaded radio stages approximately at a constant and predetermined value over a wide range of the magnitude of the radio input to said cascaded amplifier stages, said manually adjustable means comprising means for controlling a bias voltage impressed upon an amplier stage succeeding that stage at which the output voltage level is held approximately constant.

PAUL O. FARNHAM. 

